Ink-jet recording apparatuses, which can perform recording at high speed, can perform recording on plain paper without a special fixing operation, and can be obtained at relatively low prices, have often been used for many purposes of late. Using an ink-jet head in which ink liquid chambers and nozzles communicating therewith are formed, the ink-jet recording apparatus forms an image by causing ink droplets to fly from the nozzles to adhere to a recording medium such as paper or a film by applying pressure to ink in the ink liquid chambers in accordance with image information.
In the case of performing printing on plain paper in particular using this ink-jet recording apparatus, there are explicit problems of image quality degradation characteristic of the ink-jet recording apparatus in the color reproducibility, durability, lightfastness, ink drying characteristic, feathering, color bleeding, and duplex printing characteristic of an image. Further, it is extremely difficult to perform high-speed printing on plain paper while satisfying all of these characteristics.
Usually, common ink used for ink-jet printing contains water as its principal component, and further includes a coloring agent and a wetting agent for clogging prevention, such as glycerin. The coloring agent is either dye or pigment. Conventionally, dye ink is more often used for color parts because of good color development and stability. However, the image fastness properties such as lightfastness and waterfastness (water resistance) obtained by using the dye ink are inferior to those obtained by using pigment as a coloring agent. In particular, the waterfastness is unsatisfactory in the case of using plain paper although it is possible to make a certain degree of improvement by using ink-jet recording paper having an ink absorbing layer.
Therefore, in these years, use of pigment ink, which employs an organic pigment or carbon black as a coloring agent, for plain paper printing has been studied or put into practice in order to reduce the problems of the dye ink in the case of using plain paper. Unlike dye, pigment is not soluble in water. Accordingly, pigment is usually mixed with a dispersing agent so as to be stably dispersed in water by dispersing to be employed as water-based ink. By thus using pigment, the lightfastness and waterfastness can be improved. It is difficult, however, to satisfy the lightfastness and waterfastness and other image quality characteristics at the same time. In particular, in the case of performing high-speed printing on plain paper, it is difficult to obtain high image density, sufficient color development, and sufficient color reproducibility, and characteristics such as feathering, color bleeding, the duplex printing characteristic, and the ink drying (fixation) characteristic are not satisfactory enough, either.
For example, Japanese Laid-Open Patent Applications No. 6-171072 (JP6-171072) and No. 2000-355159 (JP2000-355159) disclose recording methods that solve problems in the case of performing printing on plain paper using such pigment ink. According to the ink-jet recording method disclosed in JP6-171072, an ink containing a pigment, a polymer dispersing agent, and a resin emulsion is employed. The amount of adhesion of solid content per unit area on recording paper at the time of high-duty printing is controlled to be in an appropriate range, so that printing unevenness due to pigment agglomeration is reduced regardless of paper type, thereby obtaining image quality of high density without feathering. Further, according to the ink-jet recording method disclosed in JP2000-355159, an ink composition including a pigment and a penetrant is employed, where the pigment is a surface-treated pigment having a dispersive group on its surface so as to be dispersible in an aqueous solvent by itself. The amount of ejection of the ink composition onto the recording medium side per unit area is controlled so as to prevent generation of irregular bleeding of a printed image, and the ejected ink composition is quickly dried on the recording medium, thereby obtaining a printed image of high printing density.
According to the recording method disclosed in JP6-171072, the contact angle of the employed ink with respect to sized paper such as plain paper is extremely high, such as greater than or equal to 70°, so that there are improvements such as increased printing density and reduced feathering. However, in the case of performing printing on recording paper at high duty, as much as several tens of ng/m2 of adhesion of solid content per unit area is required, thus causing a problem in terms of the ink fixation (drying) characteristic. In particular, in the case of performing high-speed printing with multiple stacked sheets of paper, there is the problem of paper contamination due to ink transfer between paper sheets. Accordingly, this recording method is not suitable for high-speed printing. Further, there is also a problem in that the background of paper appears as white stripes in a solid part or a characters part because of the high contact angle at the time of 100% duty printing depending on a paper type. Further, adjacent printed dots remain liquid droplets because of the high contact angle, so that the problem of color bleeding is likely to occur at the boundary between colors.
On the other hand, the recording method disclosed in JP2000-355159, which uses a penetrant, is advantageous in image quality because of ink dryness (fixation). Since the problem of paper contamination due to ink transfer between paper sheets is not caused in the case of performing high-speed printing with multiple stacked paper sheets, this recording method is suitable for high-speed printing. However, since the penetrant is employed in the ink composition, feathering occurs at the time of performing printing on plain paper the same as in the case of dye ink. In the case of plain paper in particular, the ink also penetrates in the direction of the depth of the paper, so that strike-through of the ink occurs, thus making it difficult to perform duplex printing. Accordingly, it is difficult for either one of the recording methods of JP6-171072 and JP2000-355159 to obtain a high-quality image that is fully satisfactory in all of the above-described characteristics in high-speed printing on plain paper.
It is effective to employ high-viscosity ink in order to form a high-quality image on plain paper at high speed by solving the problems caused in plain paper printing by using the pigment ink. However, in the case of employing high-viscosity ink, all nozzles in a non-printing area and those of the nozzles in a printing area that are not in printing operation are clogged so as to extremely degrade image quality when the ink is dried to be higher in viscosity around the menisci of the nozzles. Further, even if the nozzles are not clogged, it may be impossible to obtain stable ink droplet ejection characteristics (droplet velocity, droplet volume, and curving in the droplet ejection direction), so that the image quality may be degraded. In order to prevent these problems, the ink-jet recording apparatus disclosed in Japanese Laid-Open Patent Application No. 2005-41039 (JP2005-41039) generates multiple ejection drive pulses and multiple non-ejection drive pulses in one print cycle. The ejection drive pulses are output in time series in order to eject liquid droplets. The non-ejection drive pulses provide a meniscus with vibration that is so slight that an ink droplet is not ejected. Each non-ejection drive pulse include a first signal that expands a pressure chamber communicating with a nozzle, a second signal that retains the expanded state of the pressure chamber after the first signal, and a third signal that contracts the pressure chamber after the third signal. Further, with respect to the ejection drive pulses, the interval between a drive pulse and its preceding drive pulse (pulse interval) is three to five times the natural frequency of the pressure chamber.
When a first signal that expands a pressure chamber communicating with a nozzle, a second signal that retains the expanded state of the pressure chamber after the first signal, and a third signal that contracts the pressure chamber after the third signal are included as a non-ejection pulse in one print cycle as shown in JP2005-41039, the signal length of the non-ejection pulse increases so as to prevent printing speed from increasing.
Further, if the pulse interval of multiple ejection drive pulses output in time series in one print cycle is three to five times the natural frequency of the pressure chamber, one print cycle, that is, the drive signal length, increases so as to reduce printing speed.